1. Field of the Invention
The present invention relates to an optical disc apparatus for recording/reproducing data on/from an optical disc such as DVD (Digital Versatile Disc), and more particularly an optical disc apparatus with a performance which enables secure reading of address information from the optical disc at an initial reading of the optical disc.
2. Description of the Related Art
It is known that this kind of optical disc apparatus can record/reproduce data on/from various optical discs such as DVD +R (Recordable), DVD +RW (Rewritable), DVD −R (Recordable) and DVD −RW (Rewritable) in addition to DVD-RAM (Random Access Memory) and DVD-ROM (Read Only Memory). Address information is preformatted and recorded on a data recording surface of each of these optical discs with wobbled grooves and lands. The address information in DVD +RW and DVD +R discs (i.e. plus R discs) is formed of a wobble signal (f1) based on the wobbled grooves and containing an ADIP (Address in Pre-groove) signal. On the other hand, the address information in DVD −RW and DVD −R discs (i.e. minus R discs) is formed of an LPP (Land Pre-Pit) signal based on pre-pits on the lands and of a wobble signal (f2) which is based on the wobbled grooves and has a frequency different from that of the wobble signal (f1) of the plus R discs.
When starting the operation of the optical disc apparatus, it automatically performs defocus adjustment such that a laser beam from an optical pickup, with an objective lens, of the optical disc apparatus is focused on a data or signal recording surface of an optical disc at a point giving a maximum RF signal which gives the best jitter performance. Before each operation of recording/reproducing data on/from the optical disc, the address information in the optical disc is required to be first read for address reproduction. When reading the disc, the aberration of the optical system forming the optical pickup such as an objective lens needs to be considered.
If the aberration of the optical system is small, the defocus position of the objective lens (relative to the optical disc) which gives the best jitter performance substantially coincides with the defocus position of the objective lens giving a maximum level of the address information, so that the address information can be read normally. However, if the optical system of the optical pickup is large, the beam spot of the laser beam irradiated on the recording surface of the optical disc is not circular, causing a distortion therein. Thus, the defocus position giving the best jitter performance is displaced or offset from the defocus position giving the maximum level of the address information. This reduces the level of the address information, which may make it impossible to read the address information for the address reproduction. If the aberration of the optical system is very large, it may even cause a disc error.
In the prior art, there are various optical disc apparatus in this field. For example, Japanese Laid-open Patent Publication 2003-99964 discloses an optical disc apparatus which uses an optical disc having multiple recording layers, and controls an objective lens. More specifically, a focus error signal is generated based on reflected light from a recording layer currently being accessed, and then a defocus amount of the objective lens obtained from a characteristic value of another signal based on this reflected light is superimposed on the focus error signal to correct the focus error signal, so as to control the objective lens. Japanese Laid-open Patent Publication 2005-158093 discloses an optical disc apparatus comprising aberration generating means for generating aberration, reproduction signal acquisition means for acquiring a reproduction signal of information based on reflected light from an optical disc having a guide groove, and groove shape signal acquisition means for acquiring a groove shape signal based on the reflected light from the guide groove. Based on the reproduction signal and the groove shape signal, the aberration generating means corrects the aberration. However, each of the optical disc apparatus of the two patent publications has a complex structure, causing high cost, and requires a long time to read the address information when reproducing the optical disc.
On the other hand, Japanese Laid-open Patent Publication Hei 11-316959 discloses an optical disc apparatus in which a center value of a focus tolerance range for address reproduction and a focus tolerance range for data reproduction is set as an optimum focus bias. Further, Japanese Laid-open Patent Publication Hei 11-161978 discloses an optical disc apparatus in which a predetermined offset amount is applied to focus servo control means so as to change the focus of a laser beam, while an error rate of data, which is reproduced from an optical disc by reproducing means, is acquired so as to achieve focusing at a focal position giving a minimum error rate.
Japanese Laid-open Patent Publication 2003-257053 discloses an optical disc apparatus in which a focus offset value giving a minimum amount of recording light with a minimum error rate of reproduced information is set as an optimum focus offset (defocus) value. However, these optical disc apparatus have drawbacks. In the apparatus of Japanese Laid-open Patent Publication Hei 11-316959, the level of the address information may not be maximum at the time of address reproduction, degrading the address information reading performance. The apparatus of Japanese Laid-open Patent Publication Hei 11-161978 does not necessarily optimize the error rate of the address information, while the apparatus of Japanese Laid-open Patent Publication 2003-257053 may not increase the signal level of the address information.
There are other optical disc apparatus to adjust the focus bias and spherical aberration. For example, Japanese Laid-open Patent Publication 2006-99928 discloses an optical disc apparatus which has a spherical aberration correction mechanism to find a combination of a spherical aberration correction value and a focus bias giving a maximum RF signal so as to record/reproduce data based on the found spherical aberration correction value and focus bias. Japanese Laid-open Patent Publication 2004-241081 discloses an optical disc apparatus having a spherical aberration correction mechanism and evaluation value generating means which uses RF amplitude values as evaluation values, and acquires an optimum evaluation value from tolerance limit points of the evaluation values so as to allow a focus bias and a spherical aberration correction value to cause or correspond to the optimum evaluation value. Japanese Laid-open Patent Publication 2006-73084 discloses an optical disc apparatus which has a spherical aberration correction mechanism, and varies a focus bias and a spherical aberration correction value so as to find an optimum focus bias from both points of view of a maximum RF signal and a maximum push-pull signal. However, the apparatus of these three patent publications require a spherical aberration correction mechanism, causing a complex and expensive structure.